Elements id have the priority in CSS since they are the most specific.
You just have to use the id:
#content li.post-item > * {
margin: 0px;
}
#content .item-description {
color: #FFF;
}
#content .item-meta {
color: #666;
}
Basically id have the priority on class which the priority on tags(p,li,ul, h1…). To override the rule, just make sure you have the priority 😉